1. Field of the Invention
The present invention relates to a projection optical system, in particular, a projection optical system having a high numerical aperture.
2. Related Background Art
Lithographic processes are commonly used in the manufacture of semiconductor elements, such as integrated circuits (ICs), LSIS, liquid crystal elements, micropatterened members and micromechanical components.
A projection exposure apparatus used for photolithography generally comprises an illumination optical system with a light source and a projection optical system. Light from the illumination optical system illuminates a reticle (a first object) having a given pattern and the projection optical system transfers an image of the reticle pattern (the first object), onto a region of a photo-sensitive substrate (a second object). The image of the reticle pattern may also be reduced in size by the projection optical system so as to produce a smaller image of the reticle pattern on the substrate.
The trend to ever smaller and more sophisticated miniaturized devices places increasingly high demands on the projection exposure systems and thus projection optical systems used for the manufacture of these devices. In order to achieve better resolution in the exposure of substrates, projection optical systems with increased numerical apertures are being developed. In addition, a wavelength of light used for exposing substrates is decreased. Both of these changes bring about a whole range of new challenges in terms of a design of a projection optical system and a choice of lens materials.
At present, illumination light of wavelengths shorter than 200 nm is predominantly used in the recently developed projection exposure systems. High quality lens materials suitable for use in projection optical system having a high numerical aperture and at a short wavelength tend to be in short supply and are generally associated with high cost.
From the state of the art, projection optical systems with numerical apertures (on a substrate side of the projection optical system) up to 0.9 are known. Due to the need of correcting aberrations in the projection optical system, low angles of incidence and deflection of imaging beams (transferring an image of the first object onto the region of the second object) are generally preferred in such systems. This generally leads to projection optical systems comprising lenses of large diameter and a long distance between the first and the second object, or reticle and substrate, respectively. The large lens diameters and the long distance are, however, generally synonymous with a high mass of the lens material and thus high cost. As a consequence, the projection optical system tends to become rather heavy.
Following the present trend and developing projection optical systems using illumination light wavelengths of shorter than 200 nm and with even higher numerical apertures whilst using the approach of increasing dimensions of the projection optical system would therefore inevitably lead to unfeasibly large dimensions of a projection optical system and lenses comprised therein.